1. Field of the Invention
The present invention relates to an optical system having a diffractive optical element, and more particularly to an optical system suited to optical apparatuses, such as film cameras, video cameras, digital cameras, telescopes, projectors, etc., in which a diffractive optical element and a refracting optical element are combined to effect achromatism well.
2. Description of Related Art
Heretofore, as one of methods for correcting chromatic aberration of an optical system, there is a method of combining two glass materials (lenses) which differ in dispersion from each other.
As against such a conventional method of combining two glass materials to diminish chromatic aberration, there is a method for diminishing chromatic aberration by providing, at a lens surface or a part of an optical system, a diffractive optical element, such as a diffraction grating, having a diffracting function, as disclosed in SPIE Vol. 1354 International Lens Design Conference (1990), Japanese Laid-Open Patent Application No. Hei 4-213421 (corresponding to U.S. Pat. No. 5,044,706), Japanese Laid-Open Patent Application No. Hei 6-324262 (corresponding to U.S. Pat. No. 5,790,321), U.S. Pat. No. 5,044,706, etc.
This method is based on the utilization of the physical phenomenon that a refractive surface and a diffractive surface in an optical system cause the behavior of chromatic aberration with respect to a ray of light of a certain reference wavelength to occur in respective opposite directions. Further, it is possible to make such a diffractive optical element have an aspheric-lens-like effect by varying the period of the periodic structure thereof, thereby greatly effectively lowering aberration.
Here, while, in the case of refraction, one ray of light remains being one ray of light even after being refracted, one ray of light, in the case of diffraction, is divided into a number of rays of various orders after being diffracted. Therefore, in a case where a diffractive optical element is used in an optical system, it is necessary that the grating structure of the diffractive optical element is decided in such a manner that light fluxes included in a useful wavelength region concentrate on one particular order (hereinafter referred to also as a design order), and it is necessary that the diffractive optical element has a diffraction efficiency excellent over the entire image plane.
With regard to the diffraction grating, there is proposed, in Japanese Laid-Open Patent Application No. Hei 10-268115 (corresponding to U.S. Pat. No. 5,995,286), an optical system using a diffraction grating of the blazed shape to aim at the evenness of the diffraction efficiency over the entire observation image plane.
In the above Japanese Laid-Open Patent Application No. Hei 10-268115, there are disclosed a Keplerian viewfinder optical system arranged such that a diffraction grating of the blazed shape in which the height of a grating part at a marginal area of the diffraction grating is less than the depth of a grating part at a central area, around an optical axis, of the diffraction grating is used to make the diffraction efficiency at the central area approximately equal to that at the marginal area, and a Keplerian viewfinder optical system arranged such that a diffraction grating of the blazed shape in which a non-effective surface (a surface having no diffracting function in the diffraction grating and corresponding to a side surface of the diffraction grating) in a central area around an optical axis is formed as a part of a cylindrical surface and a non-effective surface of a marginal area is formed as a part of a conical surface is used to prevent the shading of a ray of light at the non-effective surface in the marginal area.
In the Keplerian viewfinder optical systems as proposed in the above Japanese Laid-Open Patent Application No. Hei 10-268115, an on-axial light flux and a most off-axial light flux are separate from each other when passing through the position of a diffractive optical surface provided at a position relatively distant from a stop. Accordingly, this construction has such a characteristic that, without the above state of passing-through of rays of light, it is impossible to obtain the effect of evenness of the diffraction efficiency including the shading of a ray of light.
On the other hand, in a photographing optical system to which an optical system of the invention is assumed to be applicable, as is understandable from FIGS. 1, 2 and 3, which are used for the description of embodiments of the invention, an on-axial light flux and an off-axial light flux are relatively unseparate from each other in the interior of the optical system. Accordingly, even on a lens surface distant from a stop, the area of passing-through of the on-axial light flux and that of the off-axial light flux have a tendency to overlap each other.
Therefore, even if the structural arrangement disclosed in the above Japanese Laid-Open Patent Application No. Hei 10-268115 is applied to a photographic lens as it stands, it is difficult to obtain a diffraction efficiency excellent concurrently with respect to both the on-axial light flux and the off-axial light flux.
In particular, in the case of a photographic lens having a relatively large aperture, since the respective areas, on which an on-axial light flux and an off-axial light flux are made incident, of a lens surface having a diffraction grating provided thereon overlap each other greatly, the above-mentioned difficulty increases.
It is an object of the invention to provide an optical system having such high optical performance that, when effecting achromatism by combining a diffractive optical element and a refractive optical element, a diffraction efficiency excellent over the entire image plane can be obtained even if light fluxes which are to reach respective positions of the image plane overlap each other greatly on a diffractive optical surface.
To attain the above object, in accordance with an aspect of the invention, there is provided an optical system, comprising a diffractive optical element having a diffraction grating provided, on a lens surface having a curvature, in a concentric-circles shape rotationally symmetrical with respect to an optical axis, wherein the sign of the curvature of the lens surface having the diffraction grating provided thereon is the same as the sign of a focal length, at a design wavelength, of a system composed of, in the optical system, a surface disposed nearest to an object side to a surface disposed immediately before the lens surface having the diffraction grating provided thereon, and is different from the sign of a distance from the optical axis to a position where a center ray of an off-axial light flux enters the lens surface having the diffraction grating provided thereon.
Here, the sign of the curvature of the lens surface is considered positive if the center of curvature exists on a light-exit side (image side) with respect to the lens surface, and is considered negative if the center of curvature exists on a light-entrance side (object side) with respect to the lens surface. Accordingly, the curvature of a lens surface convex facing the object side (concave facing the image side) has a positive sign, and the curvature of a lens surface concave facing the object side (convex facing the image side) has a negative sign. On the other hand, the sign of the distance from the optical axis to the position where a center ray of an off-axial light flux enters the lens surface having the diffraction grating provided thereon is considered positive if the position where the center ray enters the lens surface exists on a side opposite to a side from which the center ray enters the optical system with respect to the optical axis, and is considered negative if the position where the center ray enters the lens surface exists on the same side as a side from which the center ray enters the optical system with respect to the optical axis. Accordingly, the distance from the optical axis to the position where a center ray of an off-axial light flux enters the lens surface has a negative sign if the center ray enters the lens surface before crossing the optical axis, and has a positive sign if the center ray enters the lens surface after crossing the optical axis.
Further, in accordance with another aspect of the invention, there is provided an optical system, comprising a diffractive optical element having a diffraction grating provided, on a lens surface having curvature, in a concentric-circles shape rotationally-symmetrical with respect to an optical axis, wherein an apex of an imaginary cone formed by extending a non-effective surface of the diffraction grating (a side surface of the diffraction grating) is located adjacent to the center of curvature of the lens surface having the diffraction grating provided thereon.